I'm pretty biased towards season 10 since that was the first "older" season I immediately went to. I started watching the show really late into its life (I started not long after the movie). When the show started to really suck for me (remember, my standards were really different because I had no frame of reference for the older episodes), I began trying to find "older" episodes. When I first watched season 10, I legit thought it was some of the funniest television I had ever seen at that time. Some of the episodes I still get a kick out of, like the one where Homer becomes a truck driver or the one where they go to Japan.
Granted, I haven't seen much of season 10 since I started going further back into the show's life, so I'm not so sure I'd have the same opinions if I were to watch it now, but season 10 holds quite a bit of nostalgia for me.